1. Field of the Invention
The present invention relates to an optical fiber grating which is capable of suppressing shifts of a reflection wavelength, a method for manufacturing the optical fiber grating, and a fiber laser that includes the optical fiber.
2. Description of the Related Art
In an optical fiber grating, a structure (grating) in which a refractive index, a core diameter, and the like are periodically changed along the longitudinal direction (axial direction) of an optical fiber is formed. The optical fiber grating reflects light having a specific wavelength or combines with a cladding mode, and therefore, is capable of selectively causing a loss of light. In general, an optical fiber grating having a structure in which the refractive index is periodically changed is used. For example, the grating is manufactured by increasing the refractive index of a germanium-added silica glass through irradiation of ultraviolet light such as a KrF excimer laser or an Ar gas laser. The grating formed through the above-described method is capable of reflecting or transmitting light within a wavelength bandwidth having a center wavelength according to the design. The optical fiber grating is used widely in the field of an optical communication as an optical component or an optical device such as a wavelength selective filter, a gain equalizer of a light amplifier, or a wavelength dispersion compensator. FIG. 1 is a schematic configuration diagram of a fiber laser in which the optical fiber grating is applied as a reflection type wavelength selection filter and is used as an optical resonator. In a fiber laser 1 shown in the figure, a pump laser 11, a WDM coupler 12, a first grating 131, an amplification medium 14 such as a rare earth doped optical fiber, and a second grating 132 are connected to an optical fiber 15 in this order.
A center wavelength of a reflective wavelength bandwidth of the grating coincides with a Bragg wavelength that is determined based on a period of the grating structure and an effective refractive index of an optical fiber waveguide portion (optical fiber core portion). The period of the grating structure is changed according to a linear expansion coefficient of an optical fiber, the temperatures of the optical fiber itself and the surrounding environment, or the like. In addition, the effective refractive index of the optical fiber core portion is also changed according to the temperatures of the optical fiber itself and the surrounding environment. Therefore, the reflection wavelength of the grating is changed according to the temperature of the optical fiber itself or the surrounding environment.
In the general fiber laser 1 shown in FIG. 1, a laser oscillates at a wavelength region (a), as shown in FIG. 2A, in which the reflection wavelength of the first grating 131 and the reflection wavelength of the second grating 132 overlap each other. Thereby, as shown in FIG. 2B, if the structure and/or the core are changed so that the reflection wavelengths of the gratings do not overlap each other due to the above-described temperature change, the laser does not oscillate. Therefore, in the related art, in order to suppress shifts of the reflection wavelength of the grating, various methods in which a temperature compensation mechanism is provided in the optical fiber grating have been suggested (refer to Japanese Unexamined Patent Application, First Publication No. 2003-004956 and Japanese Unexamined Patent Application, First Publication No. 2001-318242).
In the fiber laser 1 shown in FIG. 1, in order to increase the oscillation efficiency of the laser and more stably perform laser oscillation, at least the first grating 131 of the first grating 131 and second grating 132 that configure the optical resonator use a grating having high reflectivity in the laser oscillation wavelength region. Here, it is ideal that the reflectivity be 99% or more. In order to increase the reflectivity of the grating, it is preferable to lengthen a periodic refractive index modulation structure portion in which the refractive index is periodically modulated or increase the refractive index modulation. However, if the periodic refractive index modulation structure portion is lengthened, the reflection wavelength bandwidth of the grating becomes narrow, and thus it is difficult to stably perform the laser oscillation at a desired oscillation wavelength. Therefore, in general, the reflectivity is increased by increasing the refractive index modulation. In order to increase the refractive index modulation, it is necessary to increase the irradiation intensity of ultraviolet light that is radiated to the optical fiber when the grating is formed or to lengthen the irradiation time.
However, if the irradiation time of the ultraviolet light is lengthened, due to fluctuation in air or a change in temperature during the irradiation, a minute deviation of the irradiation position of the ultraviolet light in the optical fiber may occur and the refractive index modulation portion may be blurred. In such a case, there are problems in that the refractive index modulation is not sufficiently increased and an optical fiber grating in which the laser oscillation is stably performed with high oscillation efficiency is not obtained.
On the other hand, when the irradiation intensity of the ultraviolet light is increased so as to obtain a sufficient refractive index modulation, in general, the irradiation of the ultraviolet light is performed at an intensity of approximately 3 mJ/mm2. However, in the grating that is formed according to the above-described condition, the reflection wavelength is changed when it is used, and there is a problem in that an optical fiber grating is obtained which cannot stably oscillate the laser. The inventors carried out a thorough investigation into the cause, and as a result, found that a portion which absorbs wave-guided light is generated in regions to which the ultraviolet light is radiated when the grating is formed, and the reflection wavelength of the grating is shifted due to the absorption portion. That is, since the wave-guided light is absorbed in the absorption portion when the fiber laser is operated, the grating itself is heated, and at this time, a degree of generation of heat is changed according to the intensity of the wave-guided light and the reflection wavelength of the grating is changed.
In contrast, as described above, a method in which the temperature compensation mechanism is provided in the optical fiber grating has been suggested in order to suppress the changes of the temperatures of the optical fiber itself and the surrounding environment. However, in fact, a method which controls the generation of heat of the grating itself is yet to be disclosed. If the generation of heat of the grating can be controlled, it is considered that the shifts of the reflection wavelength of the optical fiber grating can be sufficiently and easily suppressed.
The present invention is made in view of the above-described circumstances, and has an object of providing an optical fiber grating capable of sufficiently suppressing shifts of a reflection wavelength by suppressing generation of heat of a grating, a method for manufacturing the same, and a fiber laser that includes the optical fiber grating and is capable of stably performing laser oscillation.